Middle ear microphone aims to improve cochlear implants

U.S researchers are developing a tiny middle ear "microphone" that could remove the need for any external components on cochlear implants. Led by University of Utah engineer Darrin J. Young, the research team has produced and tested a prototype of the device which uses an accelerometer attached to the tiny bones of the middle ear to detect sound vibration.

Conventional cochlear implants use an externally worn microphone, speech processor and electromagnetic transmitter, along with an implanted receiver and stimulator that's wired to the auditory nerves. When sounds are picked up by the microphone and transmitted to the nerves via the internal stimulator, the patient hears.

While it has given hearing to hundreds of thousands of people around the world, this approach still has its drawbacks in terms of practicality, reliability and social perception.

“It’s a disadvantage having all these things attached to the outside” of the head, Young says. “Imagine a child wearing a microphone behind the ear. It causes problems for a lot of activities. Swimming is the main issue. And it’s not convenient to wear these things if they have to wear a helmet.”

While the conventional design doesn't make use of the ear canal and eardrum, Young's device does. It consists of a speech processor and transmitter implanted under the skin of the skull along with an accelerometer and a low-power silicon chip attached to the umbo (the point at which the eardrum connects to the three tiny ear bones). This enables it to detect vibration of the eardrum (as occurs in normal hearing). From there the system acts like a conventional cochlear implant, transmitting vibrations as electrical signals to electrodes in the cochlea.

The use of an accelerometer rather is a key to the design. Unlike standard microphones that use a diaphragm to detect sound vibrations, the accelerometer won't become clogged by growing tissue when implanted.

There is also a caveat - users would still have to wear a charger behind the ear while asleep to recharge the battery.

At this stage the researchers have tested the device in cadavers with success, but further work on miniaturization (the prototype is a "hefty" one-tenth by one-quarter inch in size and weighs 25 milligrams) and its ability to pick-up low-pitched sounds is required.

It was also found that the implant works best if the incus (anvil bone) is first removed surgically.

Young says tests in people are about three years away and has created this recording (with output going to a speaker rather than implanted electrodes) to demonstrate the device. Recognize the tune?

The study is a collaboration between Young and Case Western Reserve University electrical engineers Mark Zurcher and Wen Ko, along with ear-nose-throat physicians Maroun Semaan and Cliff Megerian of University Hospitals Case Medical Center.

The research is published online in the Institute of Electrical and Electronics Engineers journal Transactions on Biomedical Engineering.